The present invention relates to a machine for forming boxes from flat blanks, and in particular boxes which have at least a main containment body and if necessary a lid hinged to it.
The present invention is advantageously applied in lines for packaging products in general, for example for making boxes containing ice creams, snacks or any other type of products intended for sale.
The plants which manage such packaging lines comprise, connected to and operating in conjunction with each other, a machine which forms the boxes, that is to say, applies gum to several portions of the cardboard blanks and folds the blanks, a filling unit which feeds products into the boxes and a unit for closing and sealing the boxes containing the products.
The above-mentioned boxes, substantially have the shape of a parallelepiped and are obtained by means of a sequence of folding operations, from flat blanks made of stiff card material divided into a plurality of panels by creasing lines.
The above-mentioned blanks are stacked in a magazine, normally having a tilted axis, to facilitate extraction of the individual blanks from its base by suitable pick up means and the subsequent transfer to a forming unit with intermittent rotation about a horizontal axis.
The rotary forming unit comprises a cylindrical body supporting a plurality of radial arms separated from each other by equal angles, each comprising a supporting bar whose free end bears a mandrel substantially having the shape of a parallelepiped which acts as a die for forming the box-shaped body.
At least the outer wall of the mandrel, transversal to the axis of the bar supporting the mandrel and facing towards the blanks, has suction holes made in it for retaining the blank during rotation of the cylindrical body.
As the cylindrical body rotates, each arm in sequence stops at a first, loading station, at the pick up means, which comprise a pair of arms with suction cup elements oscillating about an axis parallel with the axis of the forming unit and of the cylindrical body, between the base of the magazine to pick up a blank and the mandrel suction wall, to which the blank is transferred.
For example, after a 120° rotation and gumming of predetermined zones of the blank, the mandrel supporting the blank stops at a second station, for folding, in which a folding counter-mandrel is drawn near to and couples with the mandrel, in such a way that the blank is folded about the mandrel and the main body for containing products is formed on the latter.
At a third station where the mandrel stops, for releasing, after a further rotation for example through 120°, the box-shaped body, with the open lid hinged to the main body, is removed from the mandrel by cutting off the suction applied by the mandrel suction wall and gravity causes it to drop onto a conveyor which feeds each box one after another to the above-mentioned filling unit and from there to the closing and sealing unit.
During the box forming step it is important that the blank is precisely positioned on the mandrel so that it is correctly folded along the creasing lines.
Moreover, the final, releasing step also requires a certain precision, to allow the conveyor to transfer the box to the filling unit and to correctly align it with that unit for easy, correct and rapid box filling.
In prior art plants of the type described, in particular at high production speeds there is an obvious worsening in the quality of the cardboard box.
The cause of that disadvantage may be attributed, first, to the incorrect position, which is also not strictly constant with the passage of time, adopted by the blanks relative to the folding mandrel after the operation performed by the pick up means.
The oscillating arm which extracts the blank from the magazine is synchronized with the movement of the mandrel to which the blank is transferred: however, at high speeds small phase displacements may occur between the two moving parts or slight misalignments between the blank and the mandrel, in particular at the moment when the blank passes from the oscillating arm to the mandrel.
This imprecision in the coupling between the blank and the mandrel affects the subsequent gumming steps, sometimes causing glue dribbling or incorrect gumming of the flaps of the blank, and affects the folding step, causing folds along lines other than the creasing lines.
Moreover, the box is released by the mandrel at a height of a few centimeters above the supporting surface, meaning that the box performs a small jump. This difference in height may cause incorrect positioning of the box relative to the supporting surface, preventing precise control of the exact position of the box on the conveyor.